1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a lamp driving device of a liquid crystal display device.
2. Description of the Related Art
Generally, liquid crystal display (LCD) devices are gradually increasing with respect to application scope due to characteristics such as low weight, thinness, and low power consumption. According to such trends, LCD devices are used in office automation equipment, audio/video devices, and other applications. On the other hand, LCD devices display a desired picture on a screen by controlling light transmittance in accordance with a video signal applied to a plurality of control switches arranged in a matrix.
An LCD needs a light source, such as a backlight unit, because it is not a self-luminous display device. A cold cathode fluorescent lamp (hereinafter, referred to as a “lamp”) is most commonly used as the light source in the backlight unit but other lamp types may be used. The lamp is a light source tube using a cold emission phenomenon in which electrons are emitted due to a strong electric field is applied to the surface of a cathode, thereby realizing low heat generation, high brightness, long life span and full colorization. A liquid crystal display device using a direct illumination type backlight unit having a plurality of lamps tends to be used for units of large size.
Referring to FIGS. 1 and 2, a lamp driving device of a general liquid crystal display device includes a liquid crystal display panel 6, a direct illumination type backlight unit having a plurality of lamps 12 to illuminate light to the liquid crystal display panel 6, an inverter substrate 30 to supply a high voltage AC waveform to the lamps 12, and a feedback substrate 50 to detect a tube current that flows in the lamps 12 to feed the detected tube current back to the inverter substrate 30.
The liquid crystal display panel 6 has a liquid crystal injected between an upper substrate 3 and a lower substrate 5; and a spacer (not shown) for sustaining a uniform distance between the upper substrate 3 and the lower substrate 5. In the upper substrate 3 of the liquid crystal display panel 6, a color filter, a common electrode, a black matrix and so on (not shown) are formed. Further, in the lower substrate 5 of the liquid crystal display panel 6, signal lines such as a data line and a gate line (not shown) are formed, and a thin film transistor is formed at an intersection of the data line and the gate line. The thin film transistor responds to a scan signal—gate pulse—from the gate line to switch the data signal that is to be transmitted from the data line to the liquid crystal cell. A pixel electrode is formed at a pixel area between the data line and the gate line. Further, a pad area, which is connected to each of the data line and the gate line, is formed at one side of the lower substrate 5, and a tape carrier package (not shown), on which a driver integrated circuit for applying a driving signal to the thin film transistor is mounted, is stuck to the pad area. The tape carrier package supplies the data signal and the scan signal from the driver integrated circuit to the data lines and the gate lines.
An upper polarizing sheet (not shown) is disposed the upper substrate 3 of the liquid crystal display panel 6, and a lower polarizing sheet (not shown) is disposed on the rear surface of the lower substrate 5. At this moment, the upper and lower polarizing sheets plays the role of extending the viewing angle of a picture that is displayed by a liquid crystal cell matrix.
The direct illumination type backlight unit is arranged in parallel and includes a plurality of lamps 12 that illuminates light to the liquid crystal display panel 6, a bottom cover 10 holding the lamps 12, a diffusion plate 16 to cover the front surface of the bottom cover 10, and optical sheets sequentially deposited on the diffusion plate 16. Each of the lamps 12 is composed of a glass tube, an inert gas inside the glass tube, and high voltage and low voltage electrodes installed at both ends of the glass tube. The inert gas is filled inside the glass tube, and a fluorescent substance is spread in the inner wall of the glass tube. In each of the lamps 12, if the AC voltage of high voltage supplied from the inverter substrate 30 applied to a high voltage electrode, an electron is emitted to collide with the inert gas inside the glass tube, thereby increasing the amount of electrons by geometric progression. The increased electrons cause electric current to flow in the inside of the glass tube, thereby exciting the inert gas Ar, Ne by the electron to generate energy, and the energy excites mercury to emit ultraviolet light. The ultraviolet light collides with the luminous fluorescent substance, which is spread over the inner wall of the glass tube, to emit visible light.
The bottom cover 10 of aluminum prevents the light leakage of the visible light emitted from each of the lamps 12 and reflects the visible light progressing to the side surface and rear surface of the lamps 12 toward the front surface, thereby improving the efficiency of the light generated from the lamps 12. For this, a reflecting sheet (not shown) is disposed on the bottom surface of the bottom cover 10 to reflect the light from the lamps 12.
The diffusion plate 16 causes the light emitted from the lamps 12 to progress toward the liquid crystal display panel 6 and to be incident from a wide range of angles. The diffusion plate 16 is a transparent resin film which is coated with a light diffusing member on both sides. The optical sheet 18 improves the efficiency of the light incident from the diffusion plate 16, thereby illuminating light onto the liquid crystal display panel 6.
The feedback substrate 50 includes a feedback circuit 52 that is connected to the low voltage electrode of each lamp 12 to detect the tube current that flows in the lamps 12. The feedback circuit 52 detects a voltage corresponding to the tube current that flows in the lamps 12, and rectifies the detected voltage with a diode, and then distributes the rectified voltage to divided-voltage resistance columns to detect the feedback signal. The feedback circuit 52 supplies the feedback signal detected through a feedback cable 54 to the inverter substrate 30.
As shown in FIG. 3, the inverter substrate 30 includes an input connector 40 connected to an external system; a plurality of inverter integrated circuit 34 to convert the lamp drive voltage supplied through the input connector 40 into an AC waveform; a main integrated. circuit 32 to switch the lamp drive voltage supplied through the input connector 40 to the inverter integrated circuits 34; a plurality of transformers 36 to convert the AC waveform from each of the inverter integrated circuits 34 into the AC waveform of high voltage; a plurality of output connectors 42 to output the AC waveform of high voltage supplied from the output terminal of a high voltage side of each transformer 36; and a plurality of ballast capacitors Ca connected between the output connectors 42 and the output terminals of the high voltage side of the transformers 36.
Each of the inverter integrated circuit 34 includes first and second switching devices (not shown) that convert the lamp drive voltage supplied from the main integrated circuit 32 into the AC waveform. In this way, each of the inverter integrated circuits 34 converts the lamp drive voltage into the AC waveform using the first and second switching devices to be supplied to each of two transformers 36 among the transformers 36. In other words, one inverter integrated circuit 34 drives two transformers 36. Further, each of the inverter integrated circuit 34 responds to the feedback signal supplied from the feedback substrate 50 through the feedback cable 54 to control the switching timing of first and second switching devices, thereby controlling the size of the AC waveform.
Each of the transformers 36 is arranged in parallel on the inverter substrate 30 and is composed of a primary winding and a secondary winding wound inside a body. Each transformer 36 induces the AC waveform supplied from each of the inverter integrated circuits 34 to the primary winding, to the secondary winding. The high voltage AC waveform induced to the secondary winding by the winding ratio between the primary winding and the secondary winding is supplied to each output terminal 42. The output connector 42 supplies the high voltage AC waveform to each high voltage electrode of the lamps 12 through wires (not shown).
Each ballast capacitor Ca limits the current of the high voltage AC waveform supplied from the transformer 36 to play the role of making the current balance of the inside of the lamp 12 uniform. The ballast capacitor Ca is mounted on the inverter substrate 30 by soldering a generally-used high voltage capacitor on the output terminals of the high voltage side of the transformers 36 and the output connectors 42. In this way, the lamp driving device of the general liquid crystal display device provides light—generated by the high voltage AC waveform supplied from the inverter substrate 30 to the lamps 12—onto the liquid crystal display panel 6 to control the amount of transmitted light, thereby displaying a desired picture on the liquid crystal display panel.
However, the lamp driving device of the general liquid crystal display device has the inverter substrate 30 with a large structure because of the ballast capacitors Ca mounted on the inverter substrate 30 to control the current balance of the inside of the lamps 12 as well as to limit the tube current of the lamps 12. Further, the lamp driving device of the general liquid crystal display device has a noise generated by a spark generated when making a measuring probe terminal in contact with or separated from the ballast capacitor Ca of high voltage to evaluate the characteristics of the inverter. The noise causes the feedback signal line of the inverter substrate 30 connected to the feedback cable 54 to make the inverter integrated circuit 34 mis-operate, thereby generating a shut-down phenomenon of the main integrated circuit 32.